The present invention relates to bolted, flanged joints for joining together shell sections to provide a tubular casing. More particularly, the present invention relates to an article and a method for reducing the incidence of stress-induced cracking in aircraft gas turbine engine casing flange joints adjacent bolt holes in the abutting flanges.
Tubular casings are often assembled by bolting together flanges carried at edges of plates or shells that when connected at their respective flanges define the casing. Such tubular casings are utilized to surround and enclose the principal components of aircraft gas turbine engines, wherein relatively thin-walled shell sections are connected together at flanged joints to define the engine outer casing. Outwardly-extending flanges are provided at edges of the shell sections, and the sections are joined together at their respective abutting flanges by flange bolts that pass through a plurality of respective aligned bolt holes provided in the flanges.
Aircraft gas turbine engine outer casings include several shell sections that are joined together, and consequently they include a number of flanged casing joints. Each flanged joint contributes to the overall weight of the engine because of the double thickness of the flange material, the presence of fillet radii at the intersections of the flanges and the casing outer surface, and the several connecting bolts at each such flanged joint. And because the minimization of engine weight consistent with flight safety is a continual goal of aircraft engine designers, making the flanges thinner and reducing the radial length of the flanges can contribute to the reduction of engine weight.
There are, however, practical, material-strength-related limits to flange thickness reduction that need to be observed in order to maintain a strong flanged joint. And for a given flange material and a given flange thickness, reduction of the radial lengths of abutting flanges is limited by the bolt hole size, because flange material is necessary beyond the perimeter of the bolt holes to maintain the strength of the flanged joint. In that regard, attempted engine weight reduction by reducing the radial extent of the flange outer periphery by moving the flange bolt holes closer to the casing outer surface is limited. The size of the connecting bolt head, nut, and washer or spacer, if any, and also the size of the fillet radius at the junction of the flange and the outer casing surface serve to limit the innermost positions of the bolt hole centers. In that regard, when the bolt holes are close to the fillet radii the outer edges of the bolt heads and the outer edges of the nuts or washers bear against the flanges at points immediately adjacent the fillet radii, thereby causing stress concentrations at those points when the flanged joint is subjected to the normal loads to which an engine outer casing is subjected during engine and aircraft operation. Those loads are predominantly tensile loads and bending loads that can cause stress-induced surface cracks to appear at the radially innermost contact area between the bolt heads and washers and the adjacent shell structure, where such stress concentrations can occur. Any such surface cracks that are initiated can then propagate within the shell material, and dependent upon the bolt hole spacing, they might also propagate from one bolt hole to the next adjoining bolt hole, to adversely affect the structural integrity of the outer casing itself.
It is therefore desirable to provide a way to minimize the propensity for stress-induced cracking of portions- of a shell structure adjacent to end flanges that are connected with flanges of adjacent shell structures by a series of connecting bolts.
Briefly stated, in accordance with one aspect of the present invention, a flanged joint connection is provided for securing together casing components to define a tubular casing having a longitudinal axis. The adjacent casing components each have respective abutting flanges with aligned openings extending through the flanges, and the flanges include a fillet at the junction of each flange and the outer surface of the respective casing component. The connection arrangement includes a bolt having a bolt head and an externally threaded bolt body that extends from the bolt head. The bolt head is in surface contact with an outwardly-facing, radially-extending surface of a first flange, and the bolt body extends through the aligned openings and outwardly beyond an outwardly-facing, radially-extending surface of a second flange that is in abutting relationship with the first flange. A nut is threadedly engaged with the threaded bolt body adjacent the outwardly-facing, radially extending surface of the second flange, and a spacer is positioned between and in surface contact with each of the nut and the outwardly-facing, radially-extending surface of the second flange. The spacer includes a substantially rectilinear edge portion positioned adjacent the flange fillet of the second flange and in substantially tangential relationship with an imaginary circle having its center coincident with the longitudinal axis of the tubular casing, for linearly distributing bearing surface stresses that result in the casing flange when the nut is tightened against the spacer.
In accordance with another aspect of the present invention, a method is provided for reducing stress-induced cracking in a bolted flange connection adjacent a bolt hole in a flange that extends outwardly from a tubular casing. The method includes providing a washer having a substantially rectilinear flat on its periphery, and orienting the washer so that the rectilinear flat is adjacent a fillet at the junction of a flange and an outer surface of the casing. The washer rectilinear flat is positioned so it is substantially tangent to an imaginary circle having its center lying on a longitudinal axis of the casing. The bolt is tightened to urge the washer against the flange for substantially line contact between the washer rectilinear flat and the flange.